java中的并发修改异常哈希图

Question

我编写以下代码，但运行时会生成并发修改异常

if( attendancePolicy.getType().equals( AttendanceConstants.EMPLOYEE_ATTENDANCE_POLICY ) ) {
    synchronized( attendancePolicy.getListEmployee() ) {
        for( EmployeeAttendancePolicy employeeAttendancePolicy : attendancePolicy.getListEmployee() ) {
            employeeInfo = employeeInfoSessionBeanLocal.findEmployeeInfoEntityByEmployeeInfoId( 
                    employeeAttendancePolicy.getEmployeeId() );

            if( employeeInfo != null ) {
                employeeAttendancePolicy.setEmpName( 
                        employeeInfo.getFirstName() + " " 
                        + employeeInfo.getMiddleName() + " "
                        + employeeInfo.getLastName() );

                company = companySessionBeanLocal.findCompanyById( employeeInfo.getCompanyId() );
                employeeAttendancePolicy.setCompanyName( company.getName() );

                department = departmentSessionBeanLocal.findDepartmentEntityByDepartmentId( 
                        employeeInfo.getDepartmentId() );
                employeeAttendancePolicy.setDepartmentName( department.getName() );
            }
            else {
                attendancePolicy.getListEmployee().remove( employeeAttendancePolicy );
            }
        }
    }
}

Answer 1

您正在迭代列表雇员集合的for循环中，从列表雇员中删除employeeAttendancePolicy 。 这是导致异常的“并发”操作。

如果要从循环内部删除某些内容，则需要恢复为“常规”迭代器循环（而不是上面的“增强”的for循环）。 大概是这样的：

Iterator<EmployeeAttendancePolicy> iter = attendancePolicy.getListEmployee();
while (iter.hasNext())
{
  EmployeeAttendancePolicy employeeAttendancePolicy = iter.next();

  //... somewhere in here when you decide to remove
  iter.remove();
}

Answer 2

ConcurrentModificationException是由以下事实引起的：您在List上使用了Iterator ，然后又在修改该List 。

请参阅Iterator Java文档页面-http://java.sun.com/j2se/1.4.2/docs/api/java/util/Iterator.html ，可以使用Iterator的remove方法来避免这种情况：

从基础集合中移除迭代器返回的最后一个元素（可选操作）。 每次调用next只能调用一次此方法。 如果在迭代进行过程中以其他方式（而不是通过调用此方法）修改了基础集合，则未指定迭代器的行为。

将代码重写为此：

if( attendancePolicy.getType().equals( AttendanceConstants.EMPLOYEE_ATTENDANCE_POLICY ) ) {
    synchronized( attendancePolicy.getListEmployee() ) {

        // Explicitly create the Iterator and loop condition
        for( Iterator<EmployeeAttendancePolicy> it = 
attendancePolicy.getListEmployee().iterator(); it.hasNext(); ) {

            // Explicitly declare the looping variable
            EmployeeAttendancePolicy employeeAttendancePolicy = it.next(); 

            employeeInfo = employeeInfoSessionBeanLocal.findEmployeeInfoEntityByEmployeeInfoId( 
                    employeeAttendancePolicy.getEmployeeId() );

            if( employeeInfo != null ) {
                employeeAttendancePolicy.setEmpName( 
                        employeeInfo.getFirstName() + " " 
                        + employeeInfo.getMiddleName() + " "
                        + employeeInfo.getLastName() );

                company = companySessionBeanLocal.findCompanyById( employeeInfo.getCompanyId() );
                employeeAttendancePolicy.setCompanyName( company.getName() );

                department = departmentSessionBeanLocal.findDepartmentEntityByDepartmentId( 
                        employeeInfo.getDepartmentId() );
                employeeAttendancePolicy.setDepartmentName( department.getName() );
            }
            else {

                // Use the Iterator's remove method to safely remove
                // thus avoiding the ConcurrentModificationException
                it.remove();

            }
        }
    }
}

Answer 3

您正在从要迭代的列表中删除项目。

解决此问题的一种方法是将要删除的项目添加到新集合中，然后在完成第一个循环后，从列表中删除那些项目。

例如类似

        synchronized (attendancePolicy.getListEmployee()) {
            Set toRemove = new HashSet();

            for(EmployeeAttendancePolicy employeeAttendancePolicy : attendancePolicy.getListEmployee()){

                employeeInfo=employeeInfoSessionBeanLocal.findEmployeeInfoEntityByEmployeeInfoId(employeeAttendancePolicy.getEmployeeId());

                if(employeeInfo!=null){
                    ...
                }else{
                    toRemove.add(employeeAttendancePolicy);
                }

            }
            attendancePolicy.getListEmployee().removeAll(toRemove);
        }

Answer 4

看一下ConcurrentHashMap和CopyOnWriteArrayList。 这些集合被设计为在多个线程在集合上工作时使用。